In the fields of, for example, food, biochemistry, and the environment, the Quartz Crystal Microbalance (hereinafter, referred to as QCM) method is used to measure, for example, presence of a specific substance and the density of such a substance. This QCM method employs a mass-measuring quartz vibrator which includes, as a major component, a quartz vibrating reed as a piezoelectric vibrating reed provided with a sensitive membrane which absorbs a specific substance. The sensitive membrane of the mass-measuring quartz vibrator has a molecular recognition function for achieving, for example, the densitometry and detection of the specific substance, and an exciting electrode is formed on the piezoelectric vibrating reed (quartz vibrating reed). For example, the detection and densitometry of a specific substance in liquid by the QCM method is carried out as follows.
A piezoelectric vibrating reed having a sensitive membrane formed thereon is placed in a solution and then oscillated until the oscillation frequency in this liquid stabilizes. Then, a substance which causes the reaction of absorbing or depositing substances in the liquid onto the sensitive membrane or the reaction of desorbing or decomposing substances on the sensitive membrane or a substance to be detected is added to the liquid, and the sensitive membrane on the piezoelectric vibrating reed is reacted with the specific substance to be measured. As a result, the mass on an exciting electrode of the piezoelectric vibrating reed increases or decreases, and the oscillation frequency of the piezoelectric vibrating reed decreases or increases accordingly. Consequently, the presence, density, and mass of the specific substance to be measured in the liquid can be obtained.
Japanese Unexamined Patent Application Publication No. 7-43284 describes an assay system for the amount of chemical substance by this QCM method.
In this method, a piezoelectric vibrating reed having a sensitive membrane formed thereon is connected to an oscillator circuit. This oscillator circuit is connected to a frequency counter. Furthermore, this frequency counter is connected to a computer. In an apparatus with the above-described structure, the piezoelectric vibrating reed is placed in a solution containing chemical substances and then oscillated. The oscillation frequency at this time is measured with the frequency counter. A change in frequency is obtained by subtracting from this measured oscillation frequency the frequency when the above-described sensor placed in a solution not containing chemical substances is oscillated, so that quantitative analysis of the chemical substances is performed.
In the conventional measuring apparatus with the above-described structure, the oscillation frequency of the piezoelectric vibrating reed is measured with a frequency counter, which separately requires a highly stable frequency oscillator source, and the frequency accuracy greatly affects the measurement accuracy. Furthermore, a highly stable and accurate frequency counter is expensive, and the conventional measuring apparatus is large.
In addition, measurement with a frequency counter requires at least one second of measuring time to increase the measurement accuracy, which is too long to detect a change in frequency in a short period of time.
Also, an oscillator circuit has temperature characteristics, and hence a change in temperature in the measurement environment greatly affects the measurement accuracy. In order to eliminate measurement errors associated with temperature characteristics, the system of measurement needs to be installed in a temperature-controlled environment, which causes the measuring system to become costly and large.
To overcome the drawbacks associated with the known art, an object of the present invention is to construct a measuring apparatus for detecting a change in frequency of a piezoelectric vibrating reed with a simple circuit and to provide a measuring method and a measuring apparatus for a mass-measuring vibrating reed which makes this measuring apparatus inexpensive and compact.